The exchange-coupling of a ferromagnetic film to an adjacent antiferromagnetic film with a resulting exchange-bias field in the ferromagnetic film was first reported by W. H. Meiklejohn and C. P. Bean, Phys. Rev. 102, 1413 (1959). While the magnetic hysteresis loop of a single ferromagnetic film is centered about zero magnetic field, a ferromagnetic/antiferromagnetic bilayer will show an asymmetric magnetic hysteresis loop that is shifted from zero magnetic field in the plane of the film by an exchange-bias field, HB. In many cases, the direction of the exchange-bias field within the plane of the film can be set during the growth of the antiferromagnetic film and is determined by the orientation of the magnetic moment of the ferromagnetic film when the antiferromagnetic film is deposited on top of the ferromagnetic film. The direction of the exchange bias field can also be changed by heating the ferromagnetic/antiferromagnetic bilayer above the so-called blocking temperature, TB, of the antiferromagnetic film. For other cases, the antiferromagnetic film is chemically ordered, and the direction of the exchange bias field is determined by the direction of the magnetic field when an annealing step orders the antiferromagnet. The detailed mechanism that determines the magnitude of the exchange-bias field is believed to arise from an interfacial interaction between the ferromagnetic and antiferromagnetic films.
Exchange-coupled structures have found several important applications, especially in magnetoresistive sensors used as read heads in magnetic recording hard disk drives.
The most common type of magnetoresistive sensor, called a “spin-valve” (SV) sensor, has a stack of layers that include two ferromagnetic layers separated by a nonmagnetic electrically-conducting spacer layer. One ferromagnetic layer, called the “pinned” layer, has its magnetization direction fixed by being exchange-coupled with an adjacent antiferromagnetic layer. The other ferromagnetic layer has its magnetization direction “free” to rotate in the presence of an external magnetic field, i.e., fields from the recorded data on the magnetic recording disk. With a sense current applied to the sensor, the rotation of the free-layer magnetization relative to the pinned-layer magnetization is detectable as a change in electrical resistance. The conventional SV magnetoresistive sensor operates with the sense current directed parallel to the planes of the ferromagnetic layers, so it is referred to as a current-in-the-plane (CIP) sensor. In a disk drive CIP-SV sensor or read head, the magnetization of the pinned layer is generally perpendicular to the plane of the disk and the magnetization of the free layer is generally parallel to the plane of the disk in the absence of an external magnetic field.
A SV type of magnetoresistive sensor has been proposed that operates with sense current perpendicular to the planes (CPP) of the ferromagnetic layers. CPP-SV sensors are described by A. Tanaka et al., “Spin-valve heads in the current-perpendicular-to-plane mode for ultrahigh-density recording”, IEEE TRANSACTIONS ON MAGNETICS, 38 (1): 84-88 Part 1 January 2002. Another type of CPP sensor is a magnetic tunnel junction (MTJ) sensor in which the nonmagnetic spacer layer is a very thin nonmagnetic insulating tunnel barrier layer. In a MTJ sensor the tunneling current perpendicularly through the ferromagnetic layers depends on the relative orientation of the magnetizations in the two ferromagnetic layers. While in a CPP-SV magnetoresistive read head the nonmagnetic spacer layer separating the pinned and free ferromagnetic layers is electrically conductive and is typically copper, in a MTJ magnetoresistive read head the spacer layer is electrically insulating and is typically alumina (Al2O3).
The most common material used for the antiferromagnetic layer to exchange-bias the pinned ferromagnetic layer in magnetoresistive sensors is a chemically-ordered Mn alloy with a tetragonal crystalline structure, such as PtMn, NiMn, IrMn, PdMn and RhMn. These alloys provide relatively high exchange-bias fields, and are described in U.S. Pat. No. 5,315,468. The Mn alloy material is initially chemically-disordered when deposited and provides no exchange-biasing, but becomes chemically-ordered when annealed, as a result of thermally-activated atomic diffusion, and then provides exchange-biasing of the pinned ferromagnetic layer.
The structure of a chemically-ordered Mn alloy with a tetragonal crystalline structure is shown in FIG. 1 for PtMn. The Pt atoms 22 and Mn atoms 24 together form a structure 20 similar to the face-centered-cubic (fcc) structure in which planes 32 of Pt atoms 22 and planes 34 of Mn atoms 24 alternate along the [001] direction. The resulting structure is termed L10 and corresponds to a super-lattice in the limit that each layer is a single atomic plane thick. An axis 26 perpendicular to atomic planes 32, 34 corresponds to the C-axis of L10 structure 20, and is parallel to the [001] direction. Axes 28 are parallel to atomic planes 32, 34 and correspond to the A-axes of the L10 structure 20. In actual devices the degree of ordered tetragonal crystallinity varies with annealing. Complete ordering is not necessary, but sufficient ordering to obtain exchange anisotropy and stability is needed for a robust device.
The use of these Mn alloys, particularly PtMn, as the antiferromagnetic layer in an exchange-coupled structure in a magnetoresistive sensor presents challenges in sensor design and fabrication. These alloys must be made relatively thick and must be annealed at relatively high temperatures. In CPP sensors, the large thickness of the PtMn antiferromagnetic layer is a disadvantage because the high resistivity of PtMn reduces the sensor magnetoresistance (the deltaR/R measurable by the sensor) for a given sense current, or requires that a relatively high sense current be used in the sensor to achieve the desired magnetoresistance. In both CIP-SV CPP sensors, high anneal temperatures may not be compatible with the sensor fabrication process.
What is needed is an exchange-coupled structure that enables the use of Mn-alloys without adversely affecting the design and fabrication of magnetoresistive sensors.